Electrophotographic elements having barrier layers

ABSTRACT

ELECTROPHOTOGRAPHIC ELEMENTS ARE DESCRIBED WHICH CONTAIN A BARRIER LAYER IMTERPOSED BETWEEN THE PHOTOCONDUCTIVE LAYER AND THE CONDUCTIVE LAYER. THIS BARRIER LAYER IS FORMED OF A COPOLYMER COMPRISING RECURRING UNITS DERIVED FROM VINYL ACETATE TOGETHER WITH RECURRING UNITS OF VINYL PYRROLIDONE AND/OR AN A,B-UNSTURATED MONOALKENOIC ACID.

United States Patent 6 3,745,005 ELECTRUIHOTOGRAPHlC ELEMENTS HAVKNG BAKER LAYERS William E. Yoerger and William J. Staudenmayer,

Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y. No Drawing. Filed Aug. 25, 1971, Ser. No. 174,956 Int. Cl. 603g /06 US. Cl. 961.5 8 Claims ABSTRACT 0F DKSCLOSURE Electrophotographic elements are described which contain a barrier layer interposed between the photoconductive layer and the conductive layer. This barrier layer is formed of a copolymer comprising recurring units derived from vinyl acetate together with recurring units of vinyl pyrrolidone and/ or an ft-unsaturated monoalkenoic acid.

This invention relates to eletrophotography and more particularly to photoconductive elements and structures useful therein and to novel barrier layers useful in said elements.

Electrophotographic imaging processes and techniques are based on the discovery that certain materials which are normally insulating become conductive during exposure to electromagnetic radiation of certain wavelengths after being electrically charged. Such materials, which may be either organic or inorganic, are termed photoconductors. They are conveniently formed into usable imageforming elements by coating a layer of the photoconductive composition, together with an electrically insulating resinous binder where necessary or desirable, onto a suitable support. Such an element will accept and retain an electrostatic charge in the absence of actinic radiation. In use, the surface of the element is charged in the dark to a uniform potential and exposed to an imagewise pattern of actinic radiation, which selectively reduces the surface potential to produce a charge pattern corresponding to the imagewise radiation pattern. The resultant charge pattern or electrostatic latent image may be developed by contacting it with suitably charged marking particles which adhere in accordance with the charge pattern, or it may be transferred to another insulating surface upon which it is developed. The particles may then be fused or fixed to the surface by known means such as heat or solvent vapor, or they may be transferred to another surface to which they may similarly be fixed, to produce a permanent reproduction of the original radiation pattern.

One type of unitary photoconductive element particularly useful in electrophotography is generally produced in a multilayer structure. Such an element is prepared by coating a layer of an insulating photoconductive composition onto a film support previously overcoated with a layer of conducting material. In addition, an insulating or barrier layer is interposed between the conducting material and the photoconductive composition.

One purpose of the barrier layer in an electrophotographic element is to reduce the charge leakage in the absence of activating radiation Such charge leakage is generally referred to as dark decay. On the other hand, a suitable barrier layer must not prevent proper charge dissipation in the presence of activating radiation. The barrier layer also helps to reduce the variation in performance upon repeated use of an element. Such a variation in performance of an electrophotographic element upon repeated use is known as charge fatigue. In essence, the function of a barrier layer is to prevent passage of charge from the conductive layer to the photoconductive insulating layer thus preventing unwanted discharge of the photoconductive layer.

However, problems are often encountered with prior electrophotographic elements of this type in that there is often considerable difliculty in obtaining good adhesion between the conducting layer and the barrier layer or between the photoconductive insulating layer and the barrier layer. Because of the lack of good adhesion between layers, many prior electrophotographic elements could not be substantially flexed without causing the layers to separate in various places.

A further problem frequently encountered with electrophotographic elements employing barrier layers of the prior art concerns incubation stability. It is commonly desired, in order to stabilize the electrophotographic properties of such elements, to heat them at an elevated temperature for an extended period of time, to facilitate removal of trace amounts of residual solvent, for example. The speed of elements containing certain known barrier layers is frequently found to be seriously reduced by such treatment. Similarly, many prior materials are found to deteriorate upon normal storage due probably to changes in temperature, relative humidity, etc.

Itis, therefore, an object of this invention to provide electrophotographic elements having new barrier layers which have improved adhesion to substrates.

It is a further object of this invention to provide novel electrophotographic elements having new barrier layers to which overcoated layers readily adhere.

Still another object of this invention is to provide novel electrophotographic elements capable of forming good quality images having low background.

It is, therefore, an object of this invention to provide electrophotographic elements which contain barrier layers and which have improved incubation stability.

These and other objects and advantages are accomplished in accordance with this invention through the use in an electrophotographic element of a barrier layer comprising a random copolymer of vinyl acetate with a member selected from vinyl pyrrolidone and nap-unsaturated monoalkenoic acid having from about 3 to about 6 carbon atoms in the alkene moiety thereof. The term monoalkenoic acid has reference to an acid having 2 less hydrogen atoms than the corresponding fully saturated or alkanoic acid of the parafiin group, and is more fully characterized in Markley, Fatty Acids, Their Chemistry, Properties, Production and Uses, New York, Interscience, 1960, 2nd edition, pp. 109 if. Included among the acids which copolymerize with vinyl acetate to form these particularly random copolymers are Z-pentenoic or ,B-ethylacrylic acid, 2-hexen-oic or fl-propylacrylic acid, crotonic acid and Z-heptenoic acid. The resinous copolymers useful in accordance with this invention are typically copolymers containing a recurring unit A h'aving structure (1) or structure (2) wherein n is an integer from O to 3, and a recurring unit B having structure (3) The copolymers from which the barrier layers of the present invention are formed can be prepared by standard addition polymerization techniques using catalysts known in the art such as ultraviolet light, peroxides, azo compounds, e.g., 2,2'-azobis-(Z-methylpropionitrile), etc. Useful copolymers of vinyl pyrrolidone contain from about 90 to about by weight of vinyl acetate and preferably only about 70 to about 30% of the vinyl acetate. The copolymers involving the c p-unsaturated monoalkenoic acids typically contain from about 0.5 to about 0.9 meq. of carboxylic acid per gram of copolymer and preferably contain from about 0.6 to about 0.8 meq. of carboxylic acid.

Suitable supporting materials for the photoconductive layers of the present invention can include a variety of electrically conducting supports, such as various papers or conventional film supports such as cellulose acetate, poly(ethylene terephthalate), polystyrene and the like having a conductive substrate thereon. An especially use ful conducting support can be prepared by coating a transparent film support material such as poly(ethylene terephthalate) with a layer containing a semiconductor such as cuprous iodide dispersed in a resin. Suitable conducting coatings also can be prepared from the sodium salt of a carboxy ester lactone of a maleic anhydride-vinyl acetate copolymer. Such conducting layers and methods for their optimum preparation and use are disclosed in Minsk U.S. Pat. 3,007,901, dated Nov. 7, 1961; Trevoy U.S. Pat. 3,245,833, dated Apr. 12, 1966, Sterman et al. U.S. Pat. 3,262,807, dated July 26, 1966, and copending Gramza and Stahly application U.S. Ser. No. 717,386, filed Mar. 29, 1968, now U.S. Pat. 3,597,272, and entitled Electrophotographic Element and Process. Additional useful conductive layers include carbon-containing layers such as conductive carbon particles dispersed in a resinous binder.

Similarly, the photoconductive layer in the present electrophotographic elements can be comprised of a variety of materials. Photoconductors suitable for use in the photoconductive layer can include a number of different photoconductors such as the following:

(A) Arylamine photoconductors including substituted and unsubstituted arylamines, diarylamines, nonpolymeric triarylamines and polymeric triarylamines such as those described in U.S. Pats. 3,240,597 of Fox, dated Mar. 15, 1966 and 3,180,730 of Neugebauer et al., dated Apr. 27, 1965;

(B) Polyarylalkane photoconductors of the types described in Noe et al. U.S. Pat. 3,274,000, dated Sept. 20, 1966, Wilson U.S. Pat. 3,542,547, dated Nov. 24, 1970, and in Sens et al. U.S. Pat. 3,542,544, dated Nov. 24, 1970;

(C) 4-diarylamino substituted chalcones of the types described in Fox U.S. Pat. 3,526,501, dated Sept. 1, 1970;

(D) Non-ionic cycloheptenyl compounds of the types described in Looker U.S. Pat. 3,533,786, dated Oct. 13, 1970;

(E) Compounds containing an NN nucleus, as described in Fox U.S. Pat. 3,542,546, dated Nov. 24, 1970;

(F) Organic compounds having a 3,3'-bis-aryl-2-pyrazoline nucleus, as described in Fox et al. U.S. Pat. 3,527,- 602, dated Sept. 8, 1970;

(G) Triarylamines in which at least one of the aryl radicals is substituted by either a vinyl radical or a vinylene radical having at least one active hydrogen-containing group, as described in Brantly et al. U.S. Pat. 3,567,- 450, dated Mar. 2, 1971;

(H) Triarylamines in which at least one of the aryl radicals is substituted by an active hydrogen-containing group, as described in copending Brantly et al. application U.S. Ser. No. 706,780, filed Feb. 20, 1968, now U.S. Pat. 3,658,520;

(I) Organo-metallic compounds having at least one aminoaryl substituent attached to a Group IVa or Group Va metal atom, as described in copending Goldman and Johnson U.S. Ser. No. 650,664, filed July 3, 1967, now U.S. Pat. 3,647,429;-

(J) Organo-metallic compounds having at least one aminoaryl substituent attached to a Group IHa metal 4 atom, as described in copending Johnson U.S. Ser. No. 755,711, filed Aug. 27, 1968, now U.S. Pat. 3,607,257;

(K) Any other organic compound which exhibits photoconductive properties.

The photoconductor is usually applied by forming a mixture with a polymeric binder material and coating the mixture over the barrier layer. The photoconductive layer can be applied by a variety of means such as spray coating, swirl coating, extrusion hopper coating, etc. Also the amount of photoconductor in the layer can be varied from about 10 to about 60 percent by weight of the total solids in the photoconductive layer.

The barrier layers of the present invention can likewise be applied in a variety of ways such as spray coating, dip coating, swirl coating, extrusion hopper coating, bead application on a continuous coating machine and the like. In addition, the coating coverage can be varied widely. Useful results are obtained at coverages of from about .03 to about .3 g./ft. based on the dry weight of the resin, 7

while best results are obtained in the coverage range of from about 0.05 to about 0.15 g./ft.

In the following examples, the support used is unsubbed poly(ethylene terephthalate) having a conductive layer of cuprous iodide in a poly(vinyl formal) binder and having a dry weight coverage of 50 mg. per square meter with a surface resistivity of approximately 5 10 ohms/ sq. The photoconductive layers used in the following examples are as follows. PC-l is prepared by dissolving 0.45 g. of the dye 4-(4-dimethylaminophenyl)-2,6-diphenylthiapyrylium perchlorate in g. of dichloromethane with stirring for 16 hours. To the resultant solution is added 9.0 g. of a bisphenol A polycarbonate (Lexan 145, General Electric Co.) with stirring for 30 minutes. Thereafter, 6.0 g. of 4,4-diethylamino-2,2'-dimethyltriphenylmethane photoconductor are added followed by 30 minutes of stirring. PC-2 is prepared in a manner identical to that of PC-1 replacing the fluoroborate salt of the dye for the perchlorate salt. PC-3 is formed by mixing 75% by weight of poly(4,4-isopropylidenebisphenoxyethyl-co-ethylene terephthalate) (Vitel 101, Goodyear Tire & Rubber Co.) with 25% by weight of 4,4'-diethylamino- 2,2-dimethyltriphenylmethane photoconductor.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 A series of photoconductive elements are prepared using the supports described above. A series of these conductive supports are overcoated with varying thicknesses of a barrier layer comprised of a copolymer of vinylpyrrolidone and vinyl acetate containing 60% by weight of vinylpyrrolidone and 40% by weight of vinyl acetate. Over each of the coated supports a photoconductive layer comprising PC-2, described above, is coated at a coverage of 1.2 g. per square foot. Each of the elements is then charged under a corona source until the surface potential, as measured by an electrometer probe, reaches about 600 volts. After charging, each element is exposed from behind a step density gray scale to a 3000 K. tungsten source. The exposure causes reduction of the surface potential of the element under each step of the gray scale from its initial value, V to some lower potential, V, the exact value of which depends upon the amount of exposure received by that area. The results of the measurements are plotted on a graph of surface potential, V, vs. log exposure for each step. The speed of each element, as indicated herein, is the numerical expression of 10 multiplied by the reciprocal of the exposure in metercandle-seconds required to reduce the approximate 600 volt charge surface potential to an absolute value of 50 volts. The speeds of the elements prepared as above for both negative and positive charging are shown in Table 1.

A first conductive support as described above is overcoated with a l-micron thick layer of poly(viny1 acetate) and overcoated with the photoconductive layer PC-l at a dry coverage of 1.2 g. per square foot to form a control elment. A second conductive support as described above is overcoated with a l-micron thick layer of a copolymer of vinylpyrrolidone-vinyl acetate as in Example 1. After preparation, each of the elements are subjected to negative corona source and the initial voltage is measured and recorded. The elements are then exposed as described in Example 1 to determine the toe speeds thereof. Next, the same two elements are incubated for 2 weeks at 80% relative humidity and 120 F. Again, the elements are charged and exposed while noting the saturation potential (V and the toe speed. The results of these tests are shown in Table 2 below.

TABLE 2 Fresh Incubated Element N 0. V0 Toe speed Vo Toe speed Control. (PVA) 582 4, 250 262 8 (PVA-VP) 575 3, 780 524 4, 110

EXAMPLE 3 A support comprising subbed poly(ethylene terephthalate) having thereon a cuprous iodide conducting layer coated at a dry weight of 12 mg. per square foot as described above is overcoated with varying coverages of the copolymer of vinylpyrrolidone and vinyl acetate of Example 1. The resultant overcoated supports are then coated with a photoconductive layer comprising PC-3 above coated at a thickness of 0.7 g. per square foot. The resultant elements are tested for the initial potential (V and measured for 100 volt and 50 volt negative toe speed. The same elements are then incubated under the conditions described in Example 2 and tested again. The results of these measurements are shown in Table 3 below.

As can be seen from Table 3, the saturation potential and toe speeds are not significantly impaired with incubation.

EXAMPLE 4 A support, as described previous to the examples, is coated with a copolymer of vinyl acetate and crotonic acid (i.e., Gelva C3-V20, a commercially available ma terial sold by Monsanto) at a coverage of 0.2 g. per square foot. Over that a photoconductive layer of PC1, coated at a thickness of 1.2 g. per square foot, is applied. The resultant element is subjected to varying incubation conditions and the initial potential and negative 50 volt toe speed are determined as in Example 1. The results of these measurements are shown in Table '4 below as compared to the results obtained using a fresh control comprised of the same photoconductive material coated directly on a 0.4 OD evaporated nickel conducting layer carried on an unsubbed poly(ethylene terephthalate) support. The results show that the elements of the invention exhibit excellent incubational stability. In addition, these films also exhibit good adhesion of the photoconductive layer to the barrier layer.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

ll. In an electrophotographic element comprising a support, a conducting layer on said support, a barrier layer contiguous with said conducting layer and a photoconductive layer comprising an organic photoconductor contiguous with said barrier layer, the improvement wherein said barrier layer comprises a copolymer prepared from vinyl acetate compolymerized with a member selected from the group consisting of:

(a) vinyl pyrrolidone; and

(b) an ans-unsaturated monoalkenoic acid having from about 3 to about 6 carbon atoms in the alkene moiety thereof.

2. An electrophotographic element as in claim 1 wherein the acid is selected from the group consisting of:

(a) crotonic acid;

(b) Z-pentenoic acid;

(c) Z-hexenoic acid;

(d) Z-heptenoic acid; and

(e) itaconic acid.

3. An electrophotographic element as described in claim 1 wherein thee onducting layer comprises a semiconductor or carbon dispersed in a binder.

4. An electrophotographic element as described in claim 1 wherein said barrier layer is comprised of a copolymer of about 30 to about 70% by weight of vinyl acetate with about 70 to about 30% by weight of vinyl pyrrolidone.

5. An electrophotographic element as described in claim 1 wherein said barrier layer is comprised of a copolymer of vinyl acetate with an v c-unsaturated monoalkenoic acid having from about 3 to about 6 carbon atoms in the alkene moiety, said copolymer containing about 0.6 to about 0.8 meq. of carboxylic acid per gram of polymer.

6. An electrophotographic element as described in claim 3 wherein said conductive layer comprises cuprous iodide.

7. An electrophotographic element as described in claim 3 wherein said conductive layer comprises a conductive carbon.

7 8 8. An electrophotographic element as described in claim 3,615,403 10/1971 Cheng 96-15 3 wherein said binder for the electrically conductive layer 3,166,525 11/ 1965 Perry 117-161 UN is a formal) resin. P

References Cited 5 4,329,433 12/1968 Japan 96-15 UNITED STATES PATENTS ROLAND E. MARTIN, IR., Primary Examiner 3,652,268 3/1972 Rowe 961.5 3,243,293 3/1966 Stockale 96-1.5 3,110,621 11/1963 Doggett et a1. 961.5 X 10 117161 UP, 161 UN, 218 

